1. Field of the Invention
The present invention relates generally to endoscopes and more specifically to endoscopes which have a solid state imaging device and an optical system that includes a zoom lens unit for transmitting images to the solid state imaging device.
2. Prior Art
Endoscopes, which are instruments used to inspect cavities or openings, have found a great number of applications in medicine and other technology. In the field of medicine, the use of endoscopes permits inspection of organs or other biological specimens for the purpose of inspecting a surgical site, sampling tissue and/or facilitating the manipulation of other surgical instruments, usually with the objective of avoiding invasive and traumatizing surgical procedures.
Older conventional endoscopes used in medicine have an objective lens unit at their distal (forward) ends which transmits an image of the area forward of the objective lens unit to the proximal (rear) end of the endoscope for viewing in an eye-piece, the image being transmitted to the eye-piece via an image forwarding means in the form of a so-called relay lens set or an optical fiber bundle unit. In more recent years, in place of the eye-piece and at least part of the image forwarding means, it has been preferred to provide a small size solid state video imaging device, such as one constituting a CCD chip, in the imaging plane of the objective lens, and applying the output of that video imaging device via a suitable electronic transmission system to a video monitor for viewing by the user. With both types of image transmitting and viewing arrangements, the surgeon can view the displayed image and use the information conveyed by that image to manipulate the endoscope and also other surgical instruments that have been inserted into the patient via another incision or opening in the patient's body. In the case of endoscopes that incorporate a solid state video imaging device, the image seen by the objective lens unit can be observed in the display provided by the video monitor with or without magnification.
A critical requirement of surgical endoscopes is that the maximum cross-sectional dimension of the endoscope must be kept quite small in keeping with the objective of avoiding invasive and traumatizing surgical procedures. However, it also is necessary that the endoscope have an illumination lumen or duct of a size that will assure adequate illumination of the surgical site being inspected. In addition it is desirable to provide an optical system in the endoscope that maximizes the extent of the surgical site that is encompassed by the image seen by the surgeon (i.e., the field of view) without any substantially detrimental loss of image resolution.
In recognition of the two-fold desire to maximize the field of view and image resolution, efforts have been made by others to provide endoscopes with a zoom lens system. Such endoscopes typically include an objective lens stage, a zoom lens stage, and a focussing lens for making certain that the image passed by the zoom lens is in focus. In the case where a solid state imaging device is used in an endoscope, the desired focus control can be achieved and maintained by shifting the solid-state imaging device along the axis of the endoscope in a direction and by an amount sufficient to achieve the desired focus control.
An example of an endoscope having a zoom lens and a movable imaging device system is disclosed by U.S. Pat. No. 4,488,039, issued 11 Dec. 1984 to Masamichi Sato et al for "Imaging System Having Vari-Focal Lens For Use In Endoscope". The endoscope disclosed by Sato et al includes a control system that is adapted to estimate or calculate how much the solid state imaging device is required to be moved as a consequence of movement of the zoom lens in order to assure that the imaging surface of the imaging device is substantially at the focal plane of the zoom lens system. In essence the arrangement disclosed in U.S. Pat. No. 4,488,039 is one in which the position of the imaging device that is required to achieve proper focussing is estimated on the basis of the position of the zoom lens. However, the Sato et al endoscope is handicapped by the fact that the process of estimating or calculating an appropriate position for the imaging device in response to each new position of the zoom lens is time-consuming, due to the fact that generating the estimated positions involves continuous processing of zoom lens position data according to high order mathematical equations, with the data processing being required to provide an estimate of how much and in what direction the solid state imaging device is required to be moved to assure that the imaging surface of the imaging device is substantially in or at the focal plane of the zoom lens system. In essence the estimating is conducted "on the fly", which appears to limit the accuracy and/or response time of the system with respect to optimizing continuous focussing during movement of the zoom lens.
U.S. Pat. No. 4,488,039 also suggests that the endoscope may be modified so as to make its control system capable of detecting changes in the position of the imaging device and then estimating an appropriate position for the zoom lens in order to achieve proper focussing of the sensed image on the imaging surface of the imaging device. That arrangement also appears to suffer from the need to estimate the appropriate position for the zoom lens unit as the imaging device is being moved.
In addition to the limitations noted above, the system disclosed by U.S. Pat. No. 4,488,039 does not embody a practical electrical mechanical design that is relatively inexpensive to manufacture and also is characterized by an efficient and reliable mode of operation.